1. Field of the Invention
The present invention relates to tire tread based on polydiene rubbers.
2. Discussion of the Related Art
The tire treads occupy a special place among the structural components of the tire. This is due in particular to the fact that the tire tread forms the narrow contact surface between vehicle and roadway. The characteristics of a vehicle during driving depend in particular on the type and quality of the tire tread. An optimum tire tread must cover an extensive requirement profile. In addition to high abrasion resistance and all-weather properties, the tire tread should also exhibit good winter properties, such as low rolling resistance. Antiskid properties, in particular on a wet roadway, are very important from the point of view of driving safety. In addition to the conventional braking with blocking, the antiblocking system (ABS), which is increasingly improved and widely used in recent years, is becoming increasingly important also for the development of new tread polymers which must be specially tailored to the particular conditions during ABS braking. EP-A 0 430 617 and EP-A 0 500 338 describe tread rubbers for improved ABS braking, containing the conventional aromatic oils as plasticizers.
The room temperature elasticity is of key importance for assessing the wet skid behavior on a laboratory scale. In the past this test has proven useful for evaluating the wet skid behavior in the case of emulsion SBR.
In order to achieve good braking values, it is necessary to use tread polymers having high damping, i.e, having a particularly low room temperature elasticity. This generally requires the preparation of polymers having a high glass transition temperature according to DE-A-37 24 871. These polymers are unsaturated, elastomeric AB block copolymers containing:
from 40 to 80% of a block A based on butadiene (having a uniformly distributed vinyl group content of from 8 to 60%), PA1 from 60 to 20% of a block B containing: PA1 from 40 to 80% of a block A based on butadiene PA1 from 60 to 20% of a block B based on PA1 where the vinyl content of the diene units, is from 75 to 90%. The butadiene content of block B is preferably from 5 to 60%. PA1 from 50 to 75% of 1,3-butadiene, PA1 from 5 to 35% of isoprene and PA1 from 5 to 25% of styrene. PA1 from 40 to 75% of a block A of butadiene and styrene and/or isoprene units having a vinyl or isopropylene group content (V) of less than 15%, PA1 from 5 to 25% of a block B of butadiene and styrene and/or isoprene units (V&gt;70%); or PA1 from 5 to 25% of a block B' of styrene, isoprene and, optionally, butadiene units (V&lt;15%); and PA1 from 20 to 55% of a block C of styrene, isoprene and, optionally, butadiene units (V&gt;70%);or PA1 the isolated B block (=terpolymer) from the AB block copolymers containing: PA1 with from 65 to 85% of 1,3-butadiene PA1 and from 35 to 15% of styrene PA1 are also preferred, where the vinyl content of the diene units, based on the total copolymer, is between 35 and 90%. PA1 from 10 to 60 phr of natural rubber and PA1 from 90 to 40 phr of integral rubber. PA1 where R.sub.1 and R.sub.2 are alkyl radicals, including, but not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl and tert-butyl. Further R.sub.1 and R.sub.2 differ from each other by having different number of carbon atoms. The sum of the carbon atoms of the two radicals R.sub.1 and R.sub.2 is preferably from 5 to 7, more preferably 6. A particularly suitable ethylene glycol ether is the compound in which R.sub.1 =ethyl and R.sub.2 =tert-butyl. The glycol ethers are obtained, for example, according to the principle of the Williamson synthesis, from a sodium alcoholate and an alkyl halide. The ethers of the formula EQU R.sub.1 --O--CH.sub.2 --CH.sub.2 --O--C(CH.sub.3).sub.3 PA1 the tetrahalides of the elements Si, Ge, Sn and Pb, in particular SiCl.sub.4 ; PA1 organic compounds of the general formula R.sub.n [SiHal.sub.3 ].sub.n, where "Hal" denotes halogen and where n=1 to 6, in particular n=1 or 2, where R is an n-valent organic radical, for example an aliphatic, cycloaliphatic or aromatic radical having 6 to 16 C atoms, including, by way of example, 1,2,4-tris(2-trichlorosilylethyl) cyclohexane, 1,8-bis(trichlorosilyl) octane and 1-(trichlorosilyl)octane; PA1 organic compounds which contain at least one group, SiHal.sub.2, such as dimethylsilyl chloride; PA1 halosilanes of the general formula Si(H).sub.m (Hal).sub.4-m where 3.gtoreq.m.gtoreq.1; and PA1 di- and trivinylbenzenes, such as 1,4-divinylbenzene.
up to 60% of butadiene, PA2 from 0 to 60% of isoprene, and PA2 from 0 to 45% of styrene, PA2 (having a uniformly distributed vinyl group content of from 8 to 60%), PA2 up to 60% of butadiene, PA2 from 0 to 60% of isoprene and PA2 from 0 to 45% of styrene, PA2 up to 60% of butadiene, PA2 from 0 to 60% of isoprene and PA2 from 0 to 45% of styrene,
and where the vinyl content of the diene units, based on total AB block, is from 75 to 90%. Percentages are expressed in weight percent unless otherwise specified.
Copolymers prepared by anionic polymerization and consisting of butadiene and styrene are also suitable. These copolymers (referred to below as solution SBR) consist of a mixture of butadiene and from 15 to 35% of styrene, where the vinyl content of the butadiene units (based on butadiene) is between 35 and 90%.
These block copolymers (integral rubber) or solution SBR have a high glass transition temperature.
In Table 1, this is realized in the case of Example 2 using a typical AB block copolymer B as a model. Compared with the base (Example 1), values of 101 and 116 are obtained with an elasticity of 8 during braking with blocking on asphalt and concrete. However, the drop during ABS braking on asphalt to 84% compared with the base is completely surprising. This means that the wet skid potential incorporated in the integral rubber molecule cannot be realized on the road under slight stresses as represented by ABS braking on asphalt at low speeds.
During braking on concrete, i.e. under higher stress, virtually the same standard is achieved, with a rating of 98%.